Letter position dyslexia is a deficit in the encoding of letter position within words. It is characterized by errors of letter migration within words, such as reading trail as trial and form as from. In order to examine whether Letter position dyslexia is domain-specific, and to assess the domain specificity of the visual analysis system, this study explored whether Letter position dyslexia extends to number reading, by testing whether individuals who have letter migrations in word reading also show migrations while reading numbers. The reading of words and numbers of 12 Hebrews-peaking individuals with developmental Letter position dyslexia was assessed. Experiment 1 tested reading aloud of words and numbers, and Experiment 2 tested same-different decisions in words and numbers. The findings indicated that whereas the participants with developmental Letter position dyslexia showed a large number of migration errors in reading words, 10 of them read numbers well, without migration errors, and not differently from the control participants. A closer inspection of the pattern of errors in words and numbers of two individuals who had migrations in both numbers and words showed qualitative differences in the characteristics of migration errors in the two types of stimuli. In word reading, migration errors appeared predominantly in middle letters, whereas the errors in numbers occurred mainly in final (rightmost) digits. Migrations in numbers occurred almost exclusively in adjacent digits, but in words migrations occurred both in adjacent and in nonadjacent letters. The results thus indicate that words can be selectively impaired, without a parallel impairment in numbers, and that even when numbers are also impaired they show different error pattern. Thus, the visual analyzer is actually an orthographic-visual analyzer, a module that is domain-specific for the analysis of words.

Neglect patients often omit or misread initial letters of single words, a phenomenon termed neglect dyslexia. Omissions of whole words on the contralesional side of the page during paragraph reading are generally considered as egocentric or space-based errors, whereas misreading of the left part of a word can be viewed as a type of stimulus-centred or word-based, neglect-related error. The research of the last decades shed light on several effects of word features (such as written word frequency, grammatical class or concreteness) that modulate the severity of neglect dyslexia. Nevertheless, almost all studies about those modulating factors were case studies and some of them have not been replicated yet. Therefore, to date we do not know how relevant such effects of different word stimuli are for a population of neglect dyslexia patients. Knowing their incidence would improve our theoretical understanding of neglect dyslexia and promote the development of standardized neglect dyslexia assessments, which are lacking so far. In particular, case studies have shown that neglect dyslexia error frequency increases systematically with word length (word length effect, WLE) while other single case studies found contrary results. Hence, the existence of the WLE in neglect dyslexia is unsettled and its incidence and significance in stroke patients is unknown. To clarify this issue we evaluated the relation between word length and the extent (number) of neglected or substituted letters within single words in neglect dyslexia (neglect dyslexia extent, NDE) in a group of 19 consecutive neglect dyslexia patients with right hemisphere lesions. We found a clear WLE in 79% (15 of 19) of our neglect dyslexia patients, as indicated by significant correlations between word length and NDE. Concurrent visual field defects had no effect on the WLE in our sample, thus showing no influence of early visual cortical processing stages on the WLE in neglect dyslexia. In conclusion, our results suggest a clear relationship between word length and reading errors in neglect dyslexia and show that the WLE is a frequent phenomenon in neglect dyslexia.

Dyslexia literally means poor (dys) with words (lexia). The individual with dyslexia has difficulty reading the words on the page; it is not a problem with comprehension. The weakness in word-reading is most notable when reading words in isolation. People with dyslexia often read better in context because they are able to use other skills and information to make up for weaknesses in word-reading. Ironically, this contextual facilitation effect means that many bright children with dyslexia remain unidentified because most reading tests conducted in schools use stories.

Keywords:

developmental dyslexia;

subtypes;

double deficit hypothesis;

taxometric analysis

Long-standing issues with the conceptualization, identification and subtyping of developmental dyslexia persist. This study takes an alternative approach to examine the heterogeneity of developmental dyslexia using taxometric classification techniques. These methods were used with a large sample of 671 children ages 6–8 who were diagnosed with severe reading disorders. Latent characteristics of the sample are assessed in regard to posited subtypes with phonological deficits and naming speed deficits, thus extending prior work by addressing whether these deficits embody separate classes of individuals. Findings support separate taxa of dyslexia with and without phonological deficits. Different latent structure for naming speed deficits was found depending on the definitional criterion used to define dyslexia. Non-phonologically based forms of dyslexia showed particular difficulty with naming speed and reading fluency.

Practitioner Points

Support for separate subtypes of dyslexia, with and without phonological deficits (PDs), indicates a need for different approaches to intervention.

A discrepancy-based criterion identifies more non-PD cases that may be missed with a response-to-intervention diagnosis.

Sound symbol correspondence and decoding measures may best distinguish cases of dyslexia with and without PDs.